Recently, there has been a trend in miniaturizing integrated circuits (ICs), requiring a high I/O (input/output) density which, in turn, requires small-size bonding pads. Such bonding pads are often formed on the active surface of a chip and define the places where the circuits of the chip are electrically connected to external devices. Bonding wires have become increasingly unpopular due to various potential problems, such as short circuits or inconsistent or inadequate bond strengths. Therefore, a flip chip technique has been introduced. According to this technique, solder bumps are formed on bonding pads of a chip, and the chip is mounted directly on a substrate by reflowing the solder bumps. The final product is often referred to as a ball grid array (BGA) or a flip chip ball grid array (FCBGA) chip or package.
To promote adhesion between the solder bump and the bonding pad, an under bump metallurgy or under bump metallization (UBM) structure is interposed between the solder bump and the bonding pad. UBM structures can also perform other functions, for example, as a barrier for preventing diffusion of the solder material into the bonding pad or even into the semiconductor material of the chip. A typical UBM structure includes several metal layers each performing a desired function.
Due to the concentration of multiple material layers of a UBM structure and a solder bump at and in the vicinity of each bonding pad, there is also a concentration of stress in this area. Such a stress, without preventive measure, may become sufficiently large to cause damage to inter-level dielectric (ILD) layers of the chip that are located immediately below or adjacent the bonding pad. Particularly sensitive to stress are low-k (low dielectric constant material) layers which are brittle and, in some applications, porous. Such low-k layers are easy to crack and/or delaminate under stress.